1. Field of the Invention
The present invention generally related to an onboard radar system for a vehicle and, more particularly, to an onboard radar system suitable to detect targets existing ahead of a vehicle.
2. Description of the Related Art
Conventionally, a radar system of a FM-CW (Frequency Modulation-Continuous Wave) type is known as disclosed in Japanese Laid-open Patent Application No.4-142486. The radar system has a transmitting antenna that transmits a transmitted wave converted from a transmitted signal. A modulator transmitting the transmitted signal is electrically connected to the transmitting antenna. The modulator changes a modulated frequency F of the transmitted signal so that the modulated frequency F repeatedly increases and decreases within a predetermined width .DELTA.F, about a predetermined frequency fm, with fixed changing rate. Hereinafter, the frequency fm is referred to as repeating frequency fm.
The radar system has a receiving antenna that receives a reflected wave of the transmitted wave. The receiving antenna converts the reflected wave to a received signal. The received signal vibrates with the same frequency as that of the reflected wave and has an output power according to the energy of the reflected wave.
The radar system has a signal processor that detects a beat frequency fb, which is equal to the difference between the modulated frequency F generated by the modulator and a received frequency F' of the received signal. The beat frequency fb includes information with respect to a relative distance RD and a relative velocity RV between a target that reflects the transmitted wave and the radar system. The signal processor detects the relative distance RD and the relative velocity RV based on the beat frequency fb.
An acceptable range of the beat frequency fb that the signal processor can accept is limited to a predetermined range. Hereinafter, the acceptable range is expressed by fmin.ltoreq.fb&lt;fmax. In this case, the radar system can detect targets that exist ahead of a vehicle and generate a beat frequency fb, which satisfies the condition fmin.ltoreq.fb.ltoreq.fmax.
The beat frequency fb increases as the relative distance RD increases. Thus, an upper limit of a dynamic range of the radar system is fixed by a condition fb=fmax. Accordingly, the upper limit of the dynamic range expands as a ratio expressed by fb/RD decreases. On the other hand, a lower limit of the dynamic range of the radar system is fixed by a condition fmin=fb. Accordingly, the lower limit of the dynamic range approaches the radar system, as the ratio expressed by fb/RD increases.
The system discussed above has a function changing the repeating frequency fm of the changing cycle of the modulated frequency F to a high level or a low level. The beat frequency fb decreases as the repeating frequency fm decreases. Therefore, the fb/RD ratio decreases when the repeating frequency fm is set to the low level as compared to when the repeating frequency fm is set to the high level. Accordingly, the radar system can expand the upper limit of the dynamic range ahead of the vehicle by fixing the repeating frequency fm to the low level. Further, the radar system can bring the lower limit of the dynamic range close thereto by fixing the repeating frequency fm to the high level.
In other words, the radar system can form both a long distance dynamic range and a short distance dynamic range by changing the repeating frequency fm to the low level or the high level. Therefore, the radar system discussed above can provide a larger dynamic range as compared to a radar system that does not have a function changing a scope of a dynamic range.
However, the dynamic range of the radar system, i.e., a total of the long distance dynamic range and the short distance dynamic range, covers only a part of an area expanding ahead of the vehicle. Accordingly, the dynamic range of the radar system is not necessarily enough.